The intracellular distribution of plasminogen activator in growing, confluent, and transformed 3T3 cells was investigated by using sucrose density gradient centrifugation. In confluent cells, which do not release plasminogen activator extracellularly, most of the activity was found in a heavy membrane fraction. In growing and transformed 3T3 cells, which do release plasminogen activator extracellularly in large amounts, most of the activity was found in lighter, plasma membrane-enriched fractions. A correlation between its presence in the light membrane fractions and the ability of the cells to release the plasminogen activator extracellularly is made.